Processing facilities, such as manufacturing plants, chemical plants and oil refineries, typically are managed using process control systems. Valves, pumps, motors, heating/cooling devices, and other industrial equipment typically perform actions needed to process materials in the processing facilities. Among other functions, the process control systems often control industrial operation in the processing facilities.
A very common problem encountered in many industries, such as the oil production and petrochemical industry, is compensating for variations in the flow rate of fluids (liquid and/or gases) coming into a particular processing unit. Such disturbances are usually common and ordinary events in the routine operation of the process.
As an example, in oil production processes, production from oil wells includes a mixture of oil, water and gas that flows from the wells to surface vessels, such as separators. The separation of phases can cause an undesirable operating condition when production flow becomes discontinuous with periods of large volume of liquid phase followed by periods of predominately gaseous phase. This phenomenon, referred to as “slugging,” causes the liquid flow into the separator to swing significantly. Slugging can lead to unstable operation of the process equipment downstream from the separators, which can shut down the production platform and result in a significant economic loss to the oil producing company.
Microprocessor-based proportional, integral and derivative (PID) controllers are commonly used for level control to reduce variations in the flow supplied to a downstream process. However, PID algorithms run by PID controllers often have two significant limitations. First, PID algorithms are typically unable to address non-linearities. Second, PID algorithms often cannot be used to specify high and low limits for liquid levels explicitly. Moreover, if the inlet flow has a large noise component, such as due to an upstream process that is noisy, control using a PID algorithm becomes increasingly ineffective.